Thermionic valve receiving circuits



18, 1945. P. K; CHATTERJEA ET AL 2,391,130

THERMIONICJVALVE RECEIVING CIRCUIT Filed May 8, 1943 F/GZ.

IP/O H2 n venturl Patented Dec. 18, 1945 THERMIONIC VALVE RECEIVING'GIBGUITS .Prafulla Kumar Chatterjea and Charles Thomas Scully, London, England, assignors to Standard Telephones and Cables Limited, London, England, a company of Great Britain Application May 8, 1943, Serial No. 486,250

In Great Britain July 9, 1942 4 Claims. (01, 250-20) The-present invention relates to improvements in radio receivers, and in particularly to arrangementsfor rendering such receivers quiescent when no signals are being received. The invention employs for this purpose thermally sensitive resistance devices known as thermistors. Thermistors have been in use for some years and are characterised by a temperature coefficient of resistance which may be either positive or negative and which is moreover many times the corresponding coefiicient for a pure metal such as copper. This property renders thermistors particularly suitable for a variety of special'applications in electric circuits.

Various different materials are available for the resistance element of a thermistor, these various materials having difierent properties in other respects; as one example, a resistance material having a high negative temperature coeflicient of resistance comprises a mixture of manganese'oxide' and nickel oxide, with or without the addition of certain other metallic oxides, the mixture being suitably heat treated.

. Thermistors have been employed in two different forms: (a) known as a directly heated thermistor and comprising a resistance element 'of the thermally sensitive resistance material provided with suitable lead-out conductors or terminals, and (b) known as an indirectly heated thermistor 1 comprising the element (a) provided in addition with a heating coil electrically insulated from the element. A directly heated thermistor is primarily intended to be controlled by the current which flows'through'it and which varies the temperature-and also the resistance accordingly. Such a thermistor will also be affected by the temperature of its surroundings and may therefore be used for thermostatic control and like purpose with or without direct heating by the current flowing through it; An indirectly heated thermistor is chiefly designed to be heated by a controlling current which flows through the heating coil'and which will usually, but. not necessarily, be different from the current which flows through the. resistance element, but this type of thermistor may also be subjected to either or b o th of the types of control applicable to a directly, heated thermistor.

Moredetailed information on the properties of thermistors will be found in'an article 'by G. L. Pearson in the Bell Laboratories Record.

are being received so that the set is quiet during these periods. This is commonly done by negatively biassing the grid of a valve in the circuit so that this valve is substantially cut-off.

. As soon as signals of reasonable or predetermined strength are reoeivedthis bias is overcome and the valve then amplifies normally. The objection to this method is that it introduces distortion.

By the adoption of a thermistor according to the present invention this arrangement may be employed without. the introduction of distor: tion to the received signals.

According to the invention, therefore, there is provided a radio receiving arrangement comprising an indirectly heated thermistor, the resistance element of which is associated with the cathode of a thermionic valve insuch a mamier that the. said valve is substantially blocked except when signals having a level exceeding a predetermined level are, received.

The invention will be better understood from the following detailed description of embodiments with reference to the accompanying drawing in which:

Fig. 1 shows a schematic circuit diagram of part of a radio receiver embodying the invention; and

Fig. 2 shows a schematic circuit diagram of another arrangement according to the invention.

In Fig.1 there is shown a detecting diode valve D and the following amplifier valve V. Signals are applied to the diode D through the input transformer IT the secondary winding of which is tuned by the condenser CI. The cathode of the diode D is biassed positively from the high tension supply, winch is connected to terminal 4', by means of the potentiometer comprispractice to provide means for reducing thesensitivityor gain of the receiver when no signals ing a constant resistance R3 and the resistance element R of an indirectly heated thermistor T, shunted by the usual by-pass condenser C4. The rectifiedsignals are obtained across the load resistance R2 which is in series with the diode, and in front of this resistance is a circuit B1, C2, C3 for by-passing the high frequency so that it is substantially excluded from R2. The rectified signals appearing across R2 are applied to the grid of valve V through the blocking condenser Cl acrossthe grid resistance R4. The cathode of the valve V is biassed positively in the usual way, by the resistance R5 provided with a by-pass condenser G5. The plate ofvalve V is connected to the high tension terminal 4 through the primary Winding of a transformer V 7 OT and the output from the valve V is taken to the next stage (not shown) from terminal 3 which is connected to the plate. C6 is a by-pass condenser for the high tension supply. The secondary Winding of transformer OT is connected to the heating coil r of the thermistor T.

When no signals are arriving through the input transformer IT, the resistances R and R3 are chosen so that thecathode of the-diode Disgiven a suitable positive potential. Accordingly signals at a low level are unable to overcome this potential and no effect is produced. When however, signals at a level high enough to overcome this positive potential are received the diode begins to rectify and a rectifiedi voltage is obtained across the resistance R2. This is-amplified gin thevalve V and some of the amplified current is transmitted through the transformer OT and heats the thermistor T. The thermistor is chosen to have a negative temperature coefficient; of. resistance, and accordingly its resistance decreases and reduces the positive potential applied to the cathode of the diodeD. This will, in turn, increase thesignal current amplified by the valve V which will still further reduce the potential of the cathode. The arrangement will finally stabilise at a. suitable point determined by the chosen values. of the resistances R and R3, andthev incoming signals willthen beamplifiedwithout distortion because they no longer have. to overcome a large cathode potential. So long as thesesignals are continuously received the thermistor will not cool appreciably and sothe operating conditions of the diode D. will remain substantially constant. mistor will cool and the diode will vagain ultimately be cut off.

The-circuit shown in Fig. 1 differs from a Well knowncircuit only in that the resistance R isa thermistor element and not a. constantresistance and also in the provision. ofthe transformer OT for heating the thermistor.

It should be explained that substantially the same results can be obtained with a thermistor having a positive temperature coeflicient ofi resistance if the resistances R and R3 are interchanged. It can be seen that in this case when theincoming. signals begin. to .heat the thermistor theresistance R will increase and willreduce the potential applied to the cathode of-the diode, as before.

An alternative arrangement according to the invention is. shown in Fig. 2. In this case the changes of resistance ofthe thermistor are brought about through thechanges in the plate current of the-amplifying valve controlled by an automatic volume control voltage but the principles-pf operation are otherwisethe same asin Fig. 1. In Fig. 2 the diode is-replaced by a double diodeDD-which comprises two-separate cathodes and two separate plates in the same envelope. If preferred, of course, two separate diodes could be usedinstead.

The left-hand cathode and plate of the diode DD are employed in a similar manner to the diode D-in Fig. 1, and are connected. in series circuit with the input transformer IT andthe diode load R2, the by-passing arrangement RI, C2-, C3 being the same as before. The left-handcathode is biassed positively by means of a potentiometer circuit connected across the hightension supply comprising a constant resistance R6, connected this timeat the'low potential end, andthe resistance element R of an indirectly-heatedthermistor T. G8 is the by-pass condenser for-R6. The right hand cathode and plate of thediode-DD are provided for the purpose of obtaining an auto- As soon as. thesignalscease the there matic volume control voltage from the signal. For this purpose the right-hand plate is connected to the input terminal I through the blocking condenser C9, and also to the control grid of the valve V through resistances RI 0 and RI I, and to earth through resistance R9. The condenser C8 together with resistance RM: provides a bypassing: arrangement for the high: frequency.

7 The incoming signal voltage supplied through the condenser C9 produces a, negative potential at the control grid of valve V, which potential increases as the signal level increases. This biasses the valveV'so thatthe plate current and the gain of the valve are simultaneously reduced when the signal level increases. The plate of the valve V is supplied with high tension from the terminal 4 through; the anode load RIZ, and also through the heating. coil 1" of the thermistor T. Thus when no signals are being received the plate current of the valve V will be large so that the thermistor will. be. not, If it is chosen. to have a negative temperature coeflicient of; resistancc, .the resistanceR will be low so that: theipositivepotenf tial oi the left-hand. cathode ofithe: diode. DD will be large and this partyof the diodewill besubstantially cut-off. Asasoon, as incoming:v signals: of appreciable level are receivedthe; plate. current of. the valve V willbe, reduced andthe: thermistor will cool thus reducing the potential-applied to. the left-hand cathode of. the diode DD; andcthe. lefthand portion of the diode willrbecomez unblocked: in the sameway asbefore.

It will be noted: thatthexthermistor T and the correspondingseries constant resistance are re.- spectively reversed in. Figs. 1 and-.2. This isbecause in li ig.v 1' an increase. in signal. level pro.- duces an increase. in the current: which heats the thermistor whereas in Fig. 2 it: produces: a decrease.

It will. be evident. that by interchanging the positions of the resistances RiandRii in.Eig ..2aa. thermistor having a positive. temperatureacoeffi cient of resistance can be used.

It may be pointed. out thatwhilein Eig. 1 the thermistor. resistance is. controlled by the. speech signals. themselvesafter. demodulation, in Fig. 2 it. is'controlled: by a volume. control. voltage derived from the incoming signal. wave.

What is claimed: is:

1. Radio receiver quiescent circuit including adiode, meanslfor feeding received. signals thereto; meansfor biasing saidldiode to-a predetermined potential whereby blocking occurs-and: only:sig nals. exceeding said potentialcan passthrough said diode, an amplifier valve having the input thereof connected. to the output: of saiddiode and means acting to reducesaid predetermined potential whensaid signals exceed said potential, said last means including a thermistor having-a heater and a. resistance, a. fixed resistor,.means connecting. said? thermistor resistance andsaid fixed resistor in serie with. the means determining said potential andmean coupling said? heaterto the anode circuitof. said amplifier valve for transferring heating power: current from said anode circuit to said heaterinproportionto the anode current ofthe amplifier valve.-

2. Circuit according toclaim l in which the resistance. of said thermistoranda said fixed. resistor are connected. in series. across a. source of potential. supplying said? anode circuit, the oathode of said diode is connected to the common junction of said resistance and saidifixediresistor, and the anode of. said: diode -is= connected via a load; resistor to thexnegative term'inaliof said potential source, whereby passage of current through said diode causes current to flow through said anode connection, heat said thermistor and thereby reduce the blocking potential applied to said diode.

3. Circuit according to claim 1, in which said anode connection comprises a transformer having the primary thereof connectedin series with said anode and the secondary connected to the heater of said thermistor.

4. Circuit according to claim 1, in which said connection from said heater is made so as to place said heater directly in series with the anode of said amplifier valve, and also including a second fixed resistor also in series with said heater and said anode.

PRAF'ULLA KUMAR CHA'I'IERJEA. CHARLES THOMAS SCULLY. 

